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Intermetallic Compound Growth in Tin and Tin-Lead Platings Over Nickel and Its Effects on Solderability

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Intermetallic Compound Growth in Tin and Tin-Lead Platings Over Nickel and Its Effects on Solderability Intermetallic Compound Growth in Tin and Tin-Lead Platings over Nickel and Its Effects on Solderability Solder movement combined with the right time and temperature to decompose NiSnj significantly improves solderability BY J. HAIMOVICH ABSTRACT. The growth rates for stable tion. Differential scanning calorimetry control the undesirable effects. Ni-Sn intermetallic compounds (IMC) are measurements determined that NiSn3 Nickel has been successfully used as a much lower than those for Cu-Sn IMC. indeed is a metastable phase that rapidly diffusion barrier to prevent interdiffusion Therefore, Ni appears to be a good transforms into stable IMC's and free tin of Cu and Au. This success prompted choice for a diffusion barrier between Cu at temperatures above the tin melting considerations of using Ni as a barrier and Sn. However, growth of a metasta­ point. The kinetic parameters of NiSn3 between Cu and Sn to prevent growth of ble plate-like IMC is a potential cause for transformation were calculated using Cu-Sn intermetallics and to prolong the long-term solderability degradation. This data from isothermal DSC measure­ shelf life of plated parts. Initially, there IMC has an approximate composition ments, and a time-temperature-transfor­ were some indications that growth rates NiSn3, which does not correspond to any mation (T-T-T) diagram was constructed of Ni-Sn intermetallics are very low. Since of the stable Ni-Sn IMC's on the equilibri­ using these kinetics parameters. The the early seventies, there were several um phase diagram. A long-term, low- implications of the findings on solderabili­ investigations of Ni diffusion barrier per­ temperature aging study confirmed the ty, soldering techniques and accelerated formance for Sn-based platings (Refs. 1- undesirable effects of NiSn3 growth upon aging testing for tin-based platings over 4). The results obtained by these authors solderability. Consequently, the growth nickel underplating are discussed. Also indicated a rather complex behavior. rates for NiSn3 were studied as a function discussed is the work to determine the There are three IMC's in the Ni-Sn of aging temperature, lead content, and actual mechanisms of solderability deteri­ binary system, all stable at room temper­ plating type, and were found to be oration. ature: Ni3Sn4, Ni3Sn2 and Ni3Sn (Ref. 5). affected by all of these variables. Lead Early work indicated that out of these was determined to reduce the NiSn3 Introduction three intermetallics, only Ni3Sn4 is growth in matte Sn-Pb. The growth rate present. The growth rates of Ni3Sn4 at reaches a maximum between 100° and The growth of intermetallic com­ 70°C (158°F) were found to be much 140°C (212° and 284°F) and then pounds in various platings affects solder­ lower than those of Cu-Sn intermetallics decreases. This behavior is indicative of a ability, and is one of the main factors in (Refs. 2, 3), and our results confirm this — metastable phase, and so is the composi- determining the shelf life of plated com­ Fig. 1. However, together with the slow ponents. It has been known for several growth of Ni3Sn4, an extremely fast decades that the formation of certain tEHPERATURE, *C growth of an intermetallic of unknown 250 150 100 50 20 intermetallic compounds (IMC's) in tin nature was detected (Refs. 1, 3). The W3 F and tin-based platings can have an unde­ composition of this intermetallic roughly sirable effect resulting in a serious corresponds to NiSn3, which cannot be degrading of component solderability. identified with any of the intermetallics Consequently, a considerable research on the phase diagram. effort was undertaken by the plating The characteristic platelet morphology industry to understand the formation and of this compound is shown in Fig. 2. It has growth of IMC's, and, if possible, to been suggested that the extremely fast growth of this intermetallic can cause deterioration of solderability (Ref. 3). This happens when the platelets penetrate all KEY WORDS the way through the tin layer to the surface and then oxidize. Therefore, Intermetallic Growth from the practical point of view, it is Tin/Tin Lead Plating important to be able to control this unde- Platings on Nickel Solderability Testing Low-Temperature Aging Long-Term Aging /. HAIMOVICH is a Development Engineer in NiSn3 Decomposition the Materials Engineering and Research Divi­ NiSn3 Intermetallic sion, AMP, Inc., Harrisburg, Pa. NiSn3 Metastability Fig. 1 — Intermetallic growth in matte Sn on Cu Thermal Stability Paper presented at the 12th Annual Electronics and Ni substrates. Solid line — Ni; dotted line — Manufacturing Seminar, held February 18-20, Cu 1988, at China Lake, Calif. 102-sl MARCH 1989 sirable growth. At the start of our research on NiSn3, the knowledge about the compound was rather limited. We knew that it grew in significant amounts only at lower temper­ atures, below approximately 160°C (320°F). Above this temperature, only a continuous layer of Ni3Sn4 had been observed. NiSn3 grew in a large variety of platings. The growth of the compound was the largest in bright tin over bright nickel platings, and the lowest in matte Sn over sulfamate Ni (Ref. 3). Another study found that it was not possible to control the occurrence or growth rate of the compound by variation in plating param­ eters, both for tin and nickel. The occur­ rence of the compound was either elimi­ nated or greatly reduced in nonplated layers (Ref. 6). Fig. 2 — Intermetallic growth in matte Sn, Ni underplate. Aged 1.5 years at 50°C. A —X1000; This paper can be divided into four B-X5000 parts. The first part is a brief description of a long-term, low-temperature solder­ These results contradict to some extent layer, Ni3Sn4, and discontinuous "nee­ ability study for matte tin over Cu and the results of the first test, since platings dles" (actually, platelets), NiSn3. The addi­ electroplated Ni. The second part with and without underplating display tion of 10% Pb greatly reduces the presents results of our study of short- solderability of the same order. amount of NiSn3 and the addition of 40% term growth rates of NiSn3. The third part To investigate the cause of the solder­ Pb practically eliminates it. This correlates describes the work on the thermal stabil­ ability failure, all samples were cross- well with dip test results. Most likely, the ity of NiSn3. And finally, we will discuss sectioned to reveal IMC growth. Such thin layer of Ni3Sn4 does not affect the how what we learned about this interme­ cross-sections for matte Sn, 90% Sn-10% solderability at all, and all the detrimental tallic was applied to the determination of Pb and 60% Sn-40% Pb (ail over Ni effects come from NiSn3, particularly if it actual mechanism of solderability deterio­ underplating) are shown in Figs. 3A, B and grows all the way to the surface or close ration and ways to improve solderabili­ C. For matte Sn and 90% Sn-10% Pb, the to the surface. Another interesting phe­ ty. intermetallic consists of a thin continuous nomenon that was found in this study Long-Term, Low-Temperature Aging Solderability Study Table 1—Solderability Test Results (% Passed) The purpose of the study was to assess Aging Time the effects of low-temperature aging on Sample 6 Months 12 Months 18 Months 24 Months solderability of matte tin and tin-lead platings over copper and electroplated A-Aged at 50°C nickel. The variables were: 1) composi­ 100 Sn/Ni 100 57 43 0 tion-100% Sn, 90% Sn-10% Pb, 60% 90 Sn-IOPb/Ni 100 86 86 86 Sn-40% Pb (all wt-%); 2) temperature- 60 Sn-40Pb/Ni 100 100 100 100 24°C (75°F) (room temperature) and 100 Sn/Cu 86 86 29 0 90 Sn-19Pb/Cu 38 38 0 0 50°C (122°F) (maximum warehouse tem­ 60 Sn-40Pb/Cu 29 29 14 14 perature); 3) time — samples taken out for testing at 6 month intervals, up to 24 B-Aged at 24'C 100 Sn/Ni 100 months. Plating thickness was 200/x in. ± 100 75 0 90 Sn-IOPb/Ni 100 100 86 86 50, underplating (Ni) thickness was about 60 Sn-40Pb/Ni 100 100 100 100 100M in.; substrates were pure copper. 100 Sn/Cu 100 86 43 14 Solderability testing was done by two 90 Sn-IOPb/Cu 14 14 14 0 methods. First is the test procedure for 60 Sn-40Pb/Cu 75 50 25 25 estimating solderability of metallic sur­ faces, a dip test, using nonactivated rosin flux. The required coverage to pass the test is 95%. Table 1 represents results of Table 2—Meniscograph Test Results (from Ref. 7) the test. We can see that tin-lead over nickel has the best solderability. These Aging Time 24 Months results show a trend, but are somewhat Aged at 24 °C Aged at 5C °C subjective, since they depend on the Final Value Time to Final Value Time to operator's judgment. Therefore, a sec­ of Wetting Cross Zero of Wetting Cross Zero Sample Force (g) Force (g) ond method was used, the menisco- (s) (s) graph, or wetting balance (Ref. 7). Two 100 Sn/Cu -0.23 ± 0.02 — -0.29 ± 0.03 — characteristics were employed to evalu­ 100 Sn/Ni -0.14 ± 0.03 - -0.20 ± 0.03 - ate solderability: the final value of wetting 90 Sn-10 Pb/Cu 0.05 ± 0.01 2.9 ± 0.4 0.05 ± 0.01 3.1 ± 0.3 force and the time to cross zero force 90 Sn-10 Pb/Ni 0.05 ± 0.01 2.7 ± 0.2 0.02 ± 0.01 3.9 ± 0.6 line.
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